In this circuit, data at input C is amplified by IC2 and then fed to modulator IC1. IC1 g
enerates two frequencies, depending on the values of C5, ii0, R$, Rn, and R6. The frequency/, is generated if pin 9IC1 is low and fa if pin 9IC1 is high. A square wave appears at pin 13 of IC1 and is fed to Ql, an amplifier stage, that is coupled via tuned transformer T, to the ac line via CI and C2. Notice that, for safety reasons, CI and C2 must be specifically rated for the ac line voltage.